This invention relates to polymers derived from rosin and, more specifically, to the copolymer which forms between maleic anhydride and resin acids present in rosin under the influence of a free radical initiator.
It is known that rosin can be reacted at elevated temperatures, for example, 200.degree. C., with maleic anhydride. The high temperatures promote Diels-Alder and Ene reactions between maleic anhydride and certain of the resin acids present in rosin. Because these reaction products, which are called "rosin maleic anhydride adducts," result from the simple adduction of one molecule of maleic anhydride and one molecule of reactable rosin component, they are of low molecular weight, specifically, 400 g/mole. This adduction chemistry has been described in detail by researchers at the United State Gypsum Company in Smith et al., "Products of the Reaction Between Rosins and Maleic Anhydride":, J. Paint Tech., May 1969.
Rosin and rosin maleic anhydride adducts have been found to be unsuitable for many commercial applications in which materials with much higher molecular weights find utility. In order to overcome the low melting point and lack of film-forming ability inherent with the rosin and rosin maleic anhydride adducts, researchers have for many years tried to produce high molecular weight materials (&gt;400 g/mole) from rosin. A high molecular weight polymer formed from rosin would have a higher melting point and better film-forming properties than rosin or rosin adducts alone and would thus, if formulated into an ink or coating, impart increased hardness and thermal and UV stability.
The so called "polymerization" of rosin has previously been reported to occur when rosin is treated with Lewis acids or strong Bronsted acids, such as sulfuric acid, as disclosed in U.S. Pat. No. 2,107,366 issued to Morton. However, it is well known that the vast majority of the "polymer" which results from strong acid treatment of rosin consists of rosin dimer having a molecular weight of about 604 g/mole. The structure of one such dimer, formed by treatment of methyl levopimarate with p-toluenesulfonic acid, has been described by Gigante et al., J. Chem. Soc. Chem. Comm., 13:1038-9 (1986). Similar rosin "polymers" have been reported to form upon heating rosin or rosin derivatives in the presence of a substance known to decompose under the heating conditions to form free radicals. But as disclosed by Breslow in U.S. Pat. No. 2,554,487 and U.S. Pat. No. 2,554,810, the "polymers" formed under these free-radical conditions are "chiefly dimeric". No references have been found which describe the preparation of materials consisting solely of rosin which have molecular weights above that of the dimer.
The copolymerization of rosin with a reactive comonomer has heretofore proved as elusive as the homopolymerization of rosin. In U.S. Pat. No. 2,580,876, Arvin and Gitchel disclose that rosin may be reacted with styrene at high temperatures, with or without a free-radical initiator, to form "interpolymers". These "interpolymers" are ill-defined, and the extent to which rosin has actually copolymerized with styrene is not established. U.S. Pat. No. 2,311,781 to Scrutchfield discloses a composition described as a rosin-formaldehyde-maleic anhydride "conjoint condensate".
Another direction pursued by researchers desiring to make a high molecular weight rosin-containing polymer has been to modify and/or derivatize rosin in ways that make resin acid components of rosin more amenable to a polymerization process. The derivatization of rosin frequently involved forming a reactive ester of rosin. An example of this general approach can be seen in the disclosures of Ropp in U.S. Pat. No. 2,727,872 and U.S. Pat. No. 2,727,873. Ropp prepared vinyl esters of "stabilized" (hydrogenated and/or dehydrogenated)rosin and copolymerized them with reactable vinyl compounds, including vinyl chloride and vinyl stearate, in the presence of a free-radical initiator. Another noteworthy example is U.S. Pat. No. 2,639,273 in which Gould discloses that the allyl ester of stabilized rosin, but not natural rosin, may be copolymerized with styrene or maleic anhydride in the presence of a free-radical initiator. Other examples of this general approach, including U.S. Pat. No. 3,401,154, are well known in the art but cannot be considered polymerizations of rosin as the resin acid groups themselves are not participating in the polymerization, rather they are merely pendant groups. Furthermore, the conversion of rosin to a reactive ester is an expensive, low-yield process.
Still another approach toward the goal of preparing high molecular weight rosin-containing polymers consists of reacting rosin with a preexisting polymer. As disclosed in U.S. Pat. No. 2,479,516 by Rust and Canfield, poly (allyl acetate) and rosin may be combined and heated to a temperature of 250.degree. C. with concomitant collection of acetic acid. These same authors, in U.S. Pat. No. 2,447,367, disclose that the ethylene glycol monoester of rosin can be transesterified with the preformed polymer formed by the homopolymerization of ethyl acrylate. Other examples of this general approach, including U.S. Pat. No. 3,997,487 are well known.
Copolymers obtained by the polymerization of the reaction product of rosin acid partial esters of a polyol with a polymerizable vinyl monomer, such as styrene or maleic anhydride, are disclosed in U.S. Pat. No. 3,401,154 issued to MacArthur.
A composition produced by condensing a monoalkanolamine with the reaction product of a terpene and, for example, an acid anhydride, is disclosed in U.S. Pat. No. 3,043,789 to Cyba. The terpene component may be a rosin.
Despite an apparent wealth of literature relating to the polymerization and copolymerization of rosin, the inventor is not aware of any prior art which describes the production of high molecular weight copolymers formed soley between unmodified resin acids and maleic anhydride.